Major League Baseball (MLB) is the highest level of play in professional baseball in the world. More specifically, Major League Baseball ("MLB") refers to the entity that operates North America's two major leagues, the National League and the American League, by means of a joint organizational structure which has existed between them since 1903. On an organizational level, MLB effectively operates as a single "league", and as such it constitutes one of the major professional sports leagues of North America.

Major League Baseball is governed by the Major League Baseball Constitution, an agreement that has undergone several incarnations since 1876 then called the NL Constitution, with the most recent revisions being made in 2005. Major League Baseball, under the direction of its Commissioner, Bud Selig, hires and maintains the sport's umpiring crews, and negotiates marketing, labor, and television contracts. As is the case for most North American sports leagues, the 'closed shop' aspect of MLB effectively prevents the yearly promotion and demotion of teams into the Major League by virtue of their performance.

The production/multimedia wing of MLB is New York-based MLB Advanced Media, which oversees MLB.com and all 30 of the individual teams' websites. Its charter states that MLB Advanced Media holds editorial independence from the League itself, but it is indeed under the same ownership group and revenue-sharing plan. MLB Productions is a similarly-structured wing of the league, focusing on video and traditional broadcast media.

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The Major League season runs from late March or early April to late September or early October. Players and teams prepare for the season in spring training, primarily in Florida and Arizona, during February and March. Three rounds of playoffs follow the season, culminating in the World Series in late October or early November.

At the time of writing the Commissioner of Baseball, Bud Selig, has often floated the idea of international expansion and realignment of the major leagues. At the moment, however, the major leagues are each split into three divisions, and structured as listed in the table below.

In all there are 30 teams in the two leagues: 16 in the older National League ("NL") and 14 in the American League ("AL"). Each has its teams split into three divisions grouped generally by geography. They are (number of teams in each division in parenthesis): NL East (5), NL Central (5), NL West (5), AL East (5), AL Central (5) and AL West (5). The Milwaukee Brewers switched from the AL to the NL in 1998 (the only team to switch major leagues since 1900) to prevent there being an odd number of teams in any major league. The Houston Astros move to AL West in 2013 giving each league an even number of five in each division

Each team's regular season consists of 162 games, a duration established in 1961 in the American League and 1962in the National League. From 1904 to 1960, except for 1919 when 140 games were scheduled, a 154-game schedule was played. Shortened seasons were played in 1918 due to the outbreak of World War I, and in 1972, 1981, 1994 and 1995 due to player strikes and lockouts. Games are played predominantly against teams within each league through an unbalanced schedule which heavily favors intra-divisional play. In 1997 Major League Baseball introduced interleague play, which was criticized by the sport's purists but has since proven very popular with most fans.

Each year in June, Major League Baseball conducts a draft for first year players who have never signed a Major or Minor League contract. The MLB Draft is among the least followed of the professional sports drafts in the United States.

Early July marks the midway point of the season, during which a three day break is taken when the Major League Baseball All-Star Game is staged. The All-Star game pits players from the NL, headed up by the manager of the previous NL World Series team, against players from the AL, similarly managed, in an exhibition game. The 2002 contest ended in an 11-inning tie because both teams were out of pitchers, a ridiculous result which proved highly unpopular with the fans. As a result, for a two-year trial in 2003 and 2004, the league which won the game received the benefit of home-field advantage (four of the seven games of that year's World Series taking place at their home park). The 2005 contest, played in Detroit, followed this format, and it is expected that it will remain that way until the MLB says otherwise, since it has become popular with fans but has upset purists over the previous format of the two leagues alternating home-field advantage every other year. Through the 2005 season, the AL has won all three contests with this rule. The Boston Red Sox and Chicago White Sox took full advantage of the rule, with both teams winning the World Series in a 4-0 sweep in 2004 and 2005, respectively.

Since the 1970s, the eight position players for each team who take the field initially have been voted into the game by fans. The remaining position players and all of the pitchers on each league's roster were, for a long number of years, solely at the discretion of that team's manager. In 2004, however, MLB instituted a system where some reserves and pitchers were selected by a vote of MLB players, and some were selected by the manager after consulting with the Commissioner's Office. By MLB regulation, every team in the majors must have at least one designated all-star player, regardless of voting. This rule exists so that fans of every team have a player to watch for in the All Star Game.

When the regular season ends after the first Sunday in October, eight teams enter the post-season playoffs. The first six teams are each league's three division champions. The remaining two "wild-card" spots are filled by each league's team that has the best regular season record and is not a division champion. Three rounds of series of games are played to determine the champion:

World Series, a best-of-seven game series played between the champions of each league.

The matchup for the first round of the playoffs is usually 1 seed vs. 4 seed, and 2 seed vs. 3 seed, unless the 1 seed and the 4 seed are from the same division, in which case the matchup is 1 seed vs. 3 seed and 2 seed vs. 4 seed. In the first and second round of the playoffs, the better seeded team has home-field advantage.

The team belonging to the league that won the mid-season All-Star game receives home-field advantage in the World Series.

Over most of the course of Major League Baseball, steroid testing was never a major issue. However, after the BALCOsteroid scandal, which involved allegations that top baseball players had used illegal performance enhancing drugs, Major League Baseball has finally decided to issue harsher penalties for steroid users. The new policy, which was accepted by Major League Baseball players and owners, was issued at the start of the 2005 season and goes as follows:

The 1st positive test will result in a suspension of up to 10 days. The 2nd positive test will result in a suspension of 30 days. The 3rd positive test will result in a suspension of 60 days. The 4th positive test will result in a suspension of one full year. Finally, the 5th positive test will result in a penalty at the commissioner’s discretion. Players will be tested at least once per year, with the chance that several players can be tested a numerous amount of times per year. (See: List of Major League Baseball players suspended for steroids)

This program would replaces the previous steroid testing program under which, for example, no player was even suspended in 2004. Under the old policy, which was established in 2002, a first time offense would only result in treatment for the player. The new agreement makes sure that first time offenders are rightfully suspended.

In recent news, Bud Selig, the Commissioner of MLB, has proposed even tougher penalties for positive tests than the ones in place today. The new penalties that Bud Selig has proposed are a “three strikes and you’re out approach” and go as follows:

The 1st positive test would result in a 50 game suspension. The 2nd positive test would result in a 100 game suspension. Finally, the 3rd positive test would result in a lifetime suspension from MLB.

These new proposed penalties are much harsher, however they must be accepted by MLB players and owners before any changes can be made. MLB's reluctance to take a hard line on drugs (as many other sports feature far more strict testing and penalties) is widely seen as one of the main reasons why baseball has been dropped from the Olympics with effect from 2012. YEAH!!!!

In 1969, the centennial of professional baseball, a commission chartered by Major League Baseball identified the following leagues as "major leagues". The list is sometimes disputed by baseball researchers. The MLB list included the following:

In general, the official stance is that game and statistical records for these particular leagues were not kept in a consistent manner and/or those leagues did not have a significant direct impact on the major leagues.

Specifically, the following can be said of these leagues:

The NA is unquestionably recognized as the first professional league, and is the direct precursor to the NL, most of whose original eight teams came from the NA. The standard position is that the NA was a "transitional" league that was not quite up to major league standards. The NL was a wholly new entity that took the best remnants of the NA and imposed a discipline that was lacking in the failed NA.

The AL itself asserted that it was a minor league in 1900, although it was already located in most of the cities it would be operating in the following year. However, in 1900 it operated independently and did not conduct raids on major league rosters. That changed in 1901.

The Negro Leagues are the toughest call. Some historians have labeled their time the era of "shadow ball", a segregated parallel to the (all-white) major leagues. The fact that many young players were able to come into the majors in the late 1940s and early 1950s, and have immediate impact, possibly argues for major status. It could also be argued that the Negro Leagues were more properly equated to the highest levels of minor league ball, such as the Pacific Coast League. It is a debate that has no clear resolution, which is why most historians are content to simply regard them as a category unto themselves.

Conversely, some historians question whether the Union Association really qualifies as "major", because it really only had one major-league caliber team (St. Louis) and its membership was a revolving door. The Union's chief claim to major status would rest on having had some direct impact on the other majors, due to roster-raiding. None of the three "non-major" groups listed above could make that claim.